The major objective of the present proposal is to further our understanding of the regulation of arginine and urea biosynthesis in mammalian systems, and to elucidate the factors or mechanisms responsible for the coordinaton of urea synthesis with the other pathways of amino acid metabolism. To this end, the first enzymes of the urea pathway, carbamylphosphate synthetase I and ornithine transcarbamylase, will be studied to determine their role in this process. The purified enzymes will be further characterized as to their physicochemical properties in order to establish the structural features which determine the active and inactive conformational forms and macromolecular organization of the enzymes. This approach, based on structure, will provide a framework on which to interpret kinetic behavior, preferential substrate binding, and the interaction with metabolites and to evaluate the significance of these factors in the regulation of ammonia utilization. The role of N-acetyl-L-glutamate as a key factor in this process will be explored, both in terms of its mechanism of action in the activation of carbamylphosphate synthetase as well as its biosynthesis.